The last progress confirm with no doubts that the optical fibre communication sector is experiencing an incredible growth in terms of research, development, marketing and deployment of all kinds of products at any level: from large telecom to biomedical applications, passing by computer centres, consumer devices, measurement and sensing devices and so on.
The technology is progressing very quickly and large is the number of developers: from public research institutes to large private companies. A lot of subjects are new or not familiar to the CERN environment. However, they often present a big potential for CERN applications. Thus, will be vital in the future to manage keeping CERN updated on this evolution.
Optical FIbres at CERN
Trends in computer centres
Datacentre networking and cabling
The current Internet topology is a mesh in which “internet core” functions can be directly connected to final users. Furthermore the regional service provides (ISPs and IXPs) are also horizontally connected. This requires an increasing numbers of switches within the network with massive impact on the space available for them in computer centres. Thus, the current technology focuses on how to make more and more compact the switches plus on how to increase their multiple interconnections. The latter has a direct impact on cabling and powering which become critical aspects. With increasing number of ports of the switches and routers, the solutions are two:
- transition to silicon photonics (which saves power and decreases latency of the signals, between 2 ports of a router for example)
- use of high density optical cabling (which saves space with respect to copper and also increases the available bandwidth/speed). For example, when passing from 10 Gb/s to higher bit rates, the wires in the copper cables must become thicker in order to preserve the same speed over the same length, with evident consequences on their rigidity. Cat6 cables are already challenging in this respect (plus for crosstalk related aspect). The market is developing Cat8 cables. However, with this constraints, fibres become fully competitive and consequently the preferred solution for the future. Thus, it can be expected that, sooner or later, the traditional UTP cabling in data centres will be replaced by fibres, including for the final 1m connection.
To note that using multi-ribbon optical cables (current solution) will not help alone in the future to cope with density problems. In addition, the cost of fibres is becoming significant with respect to the cost of the active equipment. For these reasons, WDM is foreseen to become the standard in data centres. Some data centres use “active optical cables” as a solution for avoiding WDM. However, this is difficult to operate and manage.
Trends in specialty optical fibres
Fibre design for telecom and specialty application
Attenuation, dispersion, and non-linearities are the three limiting factors for optical fibres. They are compensated along three main guidelines:
- development of erbium doped fibres and Raman fibres allows signal amplification. Erbium is limited to 1550nm wavelength while Raman’s gain is not linear. Further developments are expected in this field.
- development continues through enhancing the design of fibre refractive index profile. Different and more complex profiles allow better control of mode dispersion, mode area, mode order…
- non-linearities are becoming more and more significant as the fibre usage reaches the Shannon limits for available bandwidth. Given to this physical limit, no more complex modulation is possible (current enhanced techniques only increase the performance of a few percents). Thus, the development of multi-core fibres (enabling spatial-division multiplexing) is seen as one of the options to cope with the need of more bandwidth. Other lines of research focus on photonic-bandgap (air core) fibres. Some institutes (like Southampton) investigate new material (Tellurite), which allow quenching the non-linearities. However, this is still to be considered as a research-specialty.
Fujikura has constructed a 12 km cable with specialty multi-core (12 cores) fibres for a total of 1000 cores in a compact cable (density of 6 cores/mm2). First results show a strong dependence of the crosstalk among cores when temperature varies (3 dB over a range from -42 to 80 C, with crosstalk improving when temperature decreases). Explanation proposed is that when T varies the cable is subject to both macro-bending and micro-bending affecting the crosstalk.
Trends in networking and high data-rate telecom transmissions
Few mode transmission in optical fibres is being explored to achieve Space
Division Multiplexing. It consists in exciting (on purpose) few modes (4 or 5,
typically), which carry in parallel different signals. This is done either in
conventional multi-mode fibres or in specialty hollow-core fibres. Research in
this sector is promising.
Another technique is using multi-core optical fibres in order to have parallel multi-channel transmission in the same fibre.
Widely-tunable lasers are becoming more and more performing for achieving high-rate wavelength multiplexing in telecom transmissions. These rely on the combination of a gaining cavity associated to two mirrors at the extremities (this whole becoming a resonant cavity) plus a filter (controlled via electronics circuits) for dynamically selecting the output wavelength. Today these lasers are able to emit in the full C-band of the optical fibre spectrum (centered around 1550nm)
Trends in silicon-photonics
This is progressing very fast. Various talks are now dedicated to the manufacturing process but also to the packaging and related issues, including quality control during production. This shows how companies are quickly approaching the stage when they will be able to mass-produce these circuits (next few years).
Trends in special optical applications
Optical wireless communication
Fraunhofer (Germany) progressed well in this field demonstrating the transmission of modulated signals up to 1Gb/s over light on a direct path. The technique is applicable also to mobile applications (cars, etc.) where a data transfer rate of 100 Mb/s was demonstrated. Definitely a technology that will generate big applications in the future.
Radio over fibre
This is quickly progressing as well with companies (notably in UK), which are already able to deploy wireless systems in buildings based on optical (instead of copper) transmission cables. These optical cables are used for transmitting radio-signals, which are then converted into electrical and irradiated via wireless directly at the antenna point by using integrated opto-electronics circuits.
- S. Meroli, A 5 Gb/s Radiation Tolerant Laser Driver in CMOS 0.13 um technology , (2012).